The present invention relates generally to devices for detecting or measuring wavelength-dependent characteristics. The present invention also relates to a system for providing wavelength control and/or stabilization of an optical system. The present invention also relates to a method of making devices with an integrated etalon structure.
Dense wavelength division multiplexing (DWDM) systems may require tight wavelength control to operate at standard channels (wavelengths). In addition, such systems may require accurate wavelength stabilization to ensure continued operation at the desired channels over the life of the product. For certain DWDM systems, the degree of wavelength stabilization accuracy may be +/xe2x88x922.5 GHz out of 193 THz, which is equivalent to +/xe2x88x9212 parts per million. Peak discrimination powers of about 3 to 7 nmxe2x88x921 may be required to achieve a locking accuracy of +/xe2x88x922.5 GHz.
U.S. Pat. No. 5,760,419 (Nabiev) refers to a wavelength meter and photodetector for use in wavelength division multiplexing (WDM) systems. The Nabiev device is formed of a distributed Bragg reflector (DBR) located in series between two photodiodes. The Nabiev device has several disadvantages. Among other things, the single-pass operation of the DBR in the Nabiev device does not lend itself to high discrimination power, as discussed in more detail below. Moreover, the Nabiev device does not operate according to a periodic transmission function. Consequently, the Nabiev device cannot be used to generate multiple signals for respective multiple wavelength regions limiting the wavelength usage of the device.
The present invention relates to an integrated semiconductor device that has first and second photodiodes and a semiconductor etalon integrally formed between the photodiodes. According to one aspect of the invention, the photodiodes are formed of alternating p- and n-type doped semiconductor materials, with photon-absorbing active material located between the doped layers. Contacts may be used to transmit signals from the photodiodes to a suitable processor for use in wavelength control and/or stabilization.
According to a preferred embodiment of the invention, the etalon is formed of at least two distributed Bragg reflectors (DBRs). The DBRs may be formed of AlGaAs or other suitable semiconductor materials. The DBRs may be attached by wafer fusion to opposite sides of the etalon center (optical cavity).
The present invention also relates to an integrated optical device for wavelength detection. The integrated device includes photodiodes for generating photoresponse signals and a discriminating semiconductor etalon located between the photodiodes. The signals may be transmitted to a suitable device for comparison. According to a preferred embodiment of the invention, the etalon selectively transmits radiation according to a periodic wavelength function.
If desired, the present invention may be incorporated into a waveguide structure, such as an optical fiber. The waveguide structure may have anti-reflective components, for example, for transmitting and receiving radiation at opposite ends.
The present invention also relates to a method of operating an optical system. The method includes the steps of transmitting radiation through a first photodetector, then through a semiconductor etalon, and then into a second photodetector. The method also includes the steps of adjusting a semiconductor etalon by applying electric current through the etalon and/or by changing the temperature of the etalon. In addition, the method includes the steps of comparing electrical signals from the photodetectors, and generating control signals based on the comparison.
According to a preferred embodiment of the invention, the control signals may be used for wavelength control, locking and/or stabilization in a dense wavelength division multiplexing (DWDM) system. The control signals may have a periodic function (with multiple peaks at predetermined resonant frequencies) to control multiple channels of the DWDM system.
The present invention also relates to a method of making integrated wavelength meters and other wavelength-responsive semiconductor devices. The method includes the steps of depositing mirrors and photodetectors on respective substrates, and then attaching the mirror/photodetector units to opposite sides of an optical cavity. Wafer fusion techniques may be used to attach the components together to form a monolithic finished product. In a preferred embodiment of,the invention, the substrates are separated from the mirrors by etching. The mirrors may be formed of semiconductor DBRs.
The resonant, multi-pass operation of the etalon structure of the present invention may be used to provide very high wavelength discrimination power. A preferred embodiment of the invention may have a discrimination power in the range of from 1 to 30 nmxe2x88x921. Such high wavelength discrimination power may be sufficient to achieve improved locking accuracy in DWDM systems.
In addition, the periodic nature of the etalon transmission function provides a robust design because it can be matched to the periodic structure of standardized channel settings.
According to another aspect of the invention, the DBRs for the etalon structure may have relatively uncomplicated structures. If one were to attempt to construct a high resolution discriminator using just a single DBR, it might be necessary to use at least 100 pairs of alternating AlGaAs materials. Substantially more pairs would be needed if InGaAsP materials were used due to the lower available xcex94n. Such complicated structures would form broadening and/or scattering mechanisms and would have significant design limitations. In the present invention, DBRs for the etalon structure may be formed with relatively few layers in each stack. Consequently, the invention provides design flexibility, for example to flow current through a mirror, or to employ thinner mirrors to facilitate etch processing.